1. Field of the Invention
The present invention relates to a relative-displacement detecting unit, and more particularly to the miniaturization of the relative-displacement detecting unit.
2. Description of the Related Art
In related art, a transducer or an encoder for detecting relative displacement is known. In a capacitance-type encoder, a transmission electrode and a reception electrode are provided on a grid (unit), and a signal electrode is provided on a scale opposing this unit. The transmission electrode and the reception electrode on the unit are capacity-coupled with the signal electrode on the scale. A drive signal is supplied to the transmission electrode, and a detection signal occurring in the reception electrode is processed by a processing circuit in correspondence with the relative position of the unit and the scale. Thus it is possible to detect the movement or the position of the unit with respect to the scale.
Meanwhile, in an induction-type encoder, the relative position is detected on the basis of the electromagnetic interaction (electromagnetic induction) between the unit and the scale. Namely, a transmission coil and a detection coil are disposed on the unit, and a scale coil is formed on the scale. As the transmission coil on the unit is driven, a magnetic flux occurs, and an induced current is generated in the scale coil on the scale by electromagnetic induction. A magnetic flux is generated by the induced current generated in the scale coil, and an induced current (induced voltage) is generated in the detection coil on the unit by the magnetic flux. Since the induced voltage varies in correspondence with the relative position of the transmission coil and the scale coil, the relative position of the unit and the scale can be detected by detecting the induced voltage generated in the detection coil.
The driving of the transmission coil is effected by using an LC resonance circuit, and the capacitance C is generally realized by using a chip capacitor. Namely, the reduction of current consumption is attained by allowing a high-frequency large current to momentarily flow across the transmission coil by making use of the chip capacitor.
However, with the configuration in which the chip capacitor is provided to utilize the LC resonance circuit, there has been a problem in that it is difficult to miniaturize the unit due to this chip capacitor.
The invention has been devised in view of the above-described problem of the conventional art. It is an object to provide a unit which can be further miniaturized by realizing an LC circuit without using a chip capacitor.
The above-mentioned object can be achieved by a relative-displacement detecting unit for detecting a relative displacement of the relative-displacement detecting unit and a scale disposed opposing the relative-displacement detecting unit, and for outputting it as an electrical signal, according to the invention, comprising: a substrate; a transmitting section formed on the substrate; and a metal film formed between the substrate and the transmitting section so as to form a capacitor connected between the transmitting section and a circuit for driving the transmitting section. A metal film is formed between the substrate and the transmitting section, and this metal film forms the capacitance C of an LC resonance circuit for supplying a drive current to the transmitting section. In other words, a chip capacitor can be substituted for by the stray capacitance of a multilayered structure, thereby making it possible to make the unit compact. In addition, since the space for providing the chip capacitor is made unnecessary, the gap between the unit and the scale can be made small correspondingly, so that it is possible to improve the detection sensitivity.
In the above-mentioned relative-displacement detecting unit, it is preferable that the metal film is a magnetic film. By disposing the magnetic film between the substrate and the transmitting section, it is possible to increase a magnetic flux generated by the transmitting section, there by making it possible to increase the detection sensitivity.
Further, in the relative-displacement detecting unit, it is preferable that the metal film is formed of two metal films disposed in opposite relation to Leach other by being spaced apart a predetermined distance therebetween. By disposing two metal films in opposite relation to each other, a capacitor is formed which is determined by the gap between the two films, the area, and the dielectric constant between the metal films. The capacitance C necessary for the LC resonance circuit can be obtained by appropriately adjusting these physical quantities. It should be noted that in a case where a built-up substrate in which an insulator is laminated on the substrate is used, this insulator may be used as an insulator between the metal films of the capacitor.
Moreover, in the above-mentioned relative-displacement detecting unit, it is preferable that the metal film is formed so as to be spaced apart a predetermined distance from the transmitting section, and the capacitor is formed between the metal film and the transmitting section. Consequently, it is possible to further simplify the construction.